This invention relates to a color image reading apparatus for photoelectrically reading a color image, and more particularly to color image reading apparatus, having image sensors arranged in a parallel, for reading a color image line by line.
A digital copying machine, which illuminates an original document, converts reflected light from the original document into an electric signal by using a photoelectric conversion device, such as a CCD, processes the electric signal and forms an image on the basis of the processed electric signal by using a laser beam printer (LBP), an LED printer or an ink jet printer, is known.
Recently, a full color digital copying machine, which can treat a full color image, has been developed.
To read a full color image of an original document, the following two methods are generally used.
(1) Reading a full color image by using a single line sensor which has a plurality of photosensitive elements provided with red filters, green filters and blue filters as shown in FIG. 1(a), so as to obtain red (R), green (G) and blue (B) image data. The assignee of the present application has proposed a reading method of this type in U.S. Pat. No 4,907,076 (CFO 4730 US).
(2) Reading a full color image by using three line sensors, arranged in parallel, which have red filters, green filters and blue filters, respectively, as shown in FIG. 1 (b), so as to obtain R, G, B image data. The assignee of the present application has proposed a reading method of this type in U.S. Pat. No 4,953,014 (CFG 108 US).
In method (1), however, it is impossible to obtain the three color image data of an identical point of the original document, because photosensitive elements having R, G, B filters cannot read the identical point.
Besides, for reading a color image of the original document of A3 size with 400 dpi (=16 dots/mm) it is necessary that the single line sensor has approximately 14,000 photosensitive elements. However, such a single line sensor having a large number of photosensitive elements has not been produced, because of disadvantages in the efficiency of producing such a sensor. Therefore, such a long line sensor is composed by connecting a plurality of short line sensors each having approximately 3,000 photosensitive elements, so a precise correction is needed for compensating the difference in the characteristics of each of the short line sensors.
On the other hand, in method (2), it is impossible to obtain the three color image data of an identical point of the original document at the same time, because three line sensors each read different lines of the original document at the same time. Therefore, outputs from line sensors must be delayed by the time corresponding to the intervals among the line sensors so as to obtain three color image data of the same line.
However, some noise, such as ghost images, occur at the color boundaries, if the speed of relative movement between the original document and the line sensors which scan the color image, is not constant.
In view of these circumstances, the assignee of the present application has proposed in USSN 863,782 (CFO 8354 US) a method in which the reflected light from the original document is color separated by using an optical device, such as a blazed diffraction grating, and three color images of the identical point of the original document are each focused on different positions. FIG. 2 shows schematic illustration of this method. According to this method, the three color image data of an identical point of the original document can be obtained at the same time, so the problems described above with regard to the method (1) or (2) do not occur.
In the color separation system using the blazed diffraction grating as shown in FIG. 2, however, the distances between a lens 22, a one-dimensional blazed diffraction grating 35 and a three-line CCD 34, the separation angles of R, G, B by the one dimensional blazed diffraction grating 35, and the intervals between the line sensors on the three-line CCD 34 which are defined in accordance with the distances and the separation angles, should be set very precisely.
If the distance between the one-dimensional blazed diffraction grating 35 and the CCD 34 is shorter than the optimum distance, the outputs of each line sensor are shown in FIG. 3(b) when the black line along the main scanning direction is read.
The output of the G-CCD becomes rather dull, as compared with the output shown in FIG. 3(a) which is the output when the distance is optimum.
Also, the outputs of the R-CCD and B-CCD become very dull because unsuitable diffracted light components from the blazed diffraction grating 35 are mixed in the R-component or the B-component light.
On the other hand, the deterioration of the outputs of CCDs is caused by not only the arrangement of the blazed diffraction grating and CCD but also the light wavelength characteristics. Accordingly, even if the blazed diffraction grating and CCD are precisely set, for example, the Modulation Transfer Functions (MTFs) of the outputs of R-CCD and B-CCD may be lower than that of G-CCD.
Therefore, the edges of the black line along the main scanning direction of CCD cannot be recognised as black image and may be recognised as color image, because an R-component signal and a B-component signal are erroneously generated at the leading part of the R-output and the trailing part of the B-output, respectively, as shown in FIG. 3.
Accordingly, if a color-pattern conversion process that recognises an image having a particular color in a full color image and converts the recognised image into a predetermined pattern, is performed, the black image will be erroneously recognised as a color image and will be converted into the predetermined pattern. So, the image subjected to the color-pattern conversion process will not be properly reproduced. The color pattern process has been proposed in USSN 843,725 and European Patent published application number 0501814 (CFO 8271 US, EP) by the assignee of the present application and, in the color pattern conversion process, an achromatic image such as white and black images should not be converted into the pattern.
Moreover, if a black letter extraction process, that recognises black letters in a full color image and reproduces the recognised black letters in a different way from other color images, is performed, the black letters will not be properly recognised and will be reproduced as a color image. The black letter extraction process has been proposed in USSN 416,587 and European Patent published application number 0363146 (CFO 6399 US EP) by the assignee of the present application.
Besides, in case of reproduction of a full color image, some deteriorations, such as the blurredness of color, the ghost and so on, may occur at the boundaries of color.